How vacuum energy is measured ?

[Chriss]

Hy. How the vacuum energy and density is measured ?

[Orodruin]

Normally what can be measured is differences in energy except for one case - Einstein's equation - which govern the geometry of space-time and therefore also the expansion of the Universe. This means that you cannot measure the vacuum energy density locally (this is a good thing, it would be quite unhealthy if you could) but instead it must be inferred from measurements at larger scales through the expansion history.

 

Now, on the other hand, differences in the vacuum energy of two setups may be measured through the Casimir effect, which results in a net force.

[Chriss]

I know that they measured vacuum energy in space in Voyager spacecraft. So they used casimir effect ?

[Orodruin]

Voyager put an upper bound on the vacuum energy density using measurements of the solar system based on how the planetary orbits behave. In order to do this, it was necessary to know the masses of the planets to great precision as well as the orbits. This limit was around 2 x 10^-17 g/cm^3. The value measured by cosmology today is around 6 x 10^-30 g/cm^3 so it should not come as a surprise that Voyager did not see a significant signal.

 

EDIT: typo

Edited May 5, 2014 by Orodruin

[Enthalpy]

Aren't there two different notions in this thread?

- The rest energy, for instance of electromagnetic waves, which is observed in the Casimir effect

- The dark energy, for which Voyager (or Pioneer maybe?) put an upper bound